Viscous well treatment fluids are commonly utilized in the drilling, completion, and treatment of subterranean formations penetrated by wellbores. A viscous well treatment fluid is generally composed of a polysaccharide or synthetic polymer in an aqueous solution which is crosslinked by metallic compounds. Examples of well treatments in which metal-crosslinked polymers are used are hydraulic fracturing, gravel packing operations, water blocking, and other well completion operations.
Hydraulic fracturing techniques are widely employed to enhance oil and gas production from subterranean formations. During hydraulic fracturing, a proppant-laden fluid is injected into a well bore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach a maximum viscosity as it enters the fracture which affects the fracture length and width. The viscosity of most fracturing fluids is generated from water-soluble polysaccharides, such as galactomannans or cellulose derivatives. Employing crosslinking agents, such as borate, titanate, or zirconium ions, can further increase the viscosity. The gelled fluid may be accompanied by a propping agent (i.e., proppant) which results in placement of the proppant within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
In formulating a fracturing fluid, several factors generally are considered. First, the fracturing fluid should preferably have sufficient viscosity to create suitable fracture geometry. Generally, most treatments are designed to obtain a specified fracture length. In addition to fracture length, it is also common to create a fracture that has a width at least three times greater than the largest proppant size. Typically, the viscosity of a fracturing fluid is designed to satisfy the requirements of both the fracture length and fracture width. Second, the viscosity of a fracturing fluid should preferably be high enough for the fluid to adequately transport the proppant from the surface to the fracture. Suitable fluids are those which can suspend the proppant with minimal settling. Third, a fracturing fluid should preferably have minimal fluid loss to the formation.
Multiple patents and publications have attempted to prepare a fracturing fluid with a combination of these desirable features.
U.S. Pat. No. 4,477,360 suggests the use of an aqueous gel containing a zirconium salt and a polyhydroxyl-containing compound. The gel is suggested for use in fracturing fluids, and has a high viscosity. The polyhydroxyl compounds have 3 to 7 carbon atoms, and a preferred compound is glycerol. Gelling agents include various polysaccharides.
U.S. Pat. No. 4,635,727 offers methods of fracturing a subterranean formation using a base guar gum gel and a crosslinking system. A preferred crosslinking system includes zirconium lactate and aluminum chlorohydrate.
U.S. Pat. No. 5,305,832 proposes methods for using crosslinked guar polymers at a pH such that the cationic charge density of the polymer is at its maximum. The pH is chosen to minimize thermal degradation and to minimize polymer gel loading. The pH varied depending on the polymer used, but were typically in the range of about 10 to about 12.
U.S. Pat. No. 5,972,850 offers an aqueous metal hydrated galactomannan gum buffered to pH 9 to 11, and methods for its use in fracturing a subterranean formation. Metal ions suggested to crosslink the galactomannan gum include boron, zirconium, and titanium ions.
U.S. Pat. No. 6,017,855 suggests methods for fracturing subterranean formations using fluids having reduced polymer loadings. The fluids contain modified polymers having randomly distributed anionic substituents. The polymers can be crosslinked to form viscous gels that are stable at low polymer concentrations. Modification of the polymers lead to lowered C* concentrations (the concentration necessary to cause polymer chain overlap).
U.S. Pat. No. 6,060,436 proposes the use of borate ion crosslinked galactomannan gums in fracturing fluids. The crosslinking is delayed by release of borate ions from a polyol complex.
SPE 29446 (1995) discusses field results of well treatment with borate-crosslinked or titanate-crosslinked systems. Performance was observed to improve with the following treatments, in increasing order of improvement: titanate-crosslinked fluids, borate-crosslinked fluids, organoborate-crosslinked fluids, and organoborate-crosslinked fluids with a guar-specific enzyme breaker. Organoborates were offered as providing stronger crosslink junctions, greater elasticity, high viscosity, and reduced polymer loadings.
SPE 36496 (1996) offers the characterization of breaker efficiency by determining the size distribution of degraded polymer fragments. Reduced viscosity was discussed as not being fully indicative of molecular weight reduction. For example, the use of oxidative breakers is capable of reducing gel viscosity, but is relatively ineffective to reduce the polymer molecular weight. Guar specific enzymes were found to provide the most efficient molecular weight reduction of crosslinked fluids.
The success of a hydraulic fracturing treatment, in part, depends upon the creation of a high-permeability fracture. The long term production of a well is directly related to fracture conductivity, which is dependent upon the fracturing fluid used during the treatment and the cleanup of the fracturing fluid after the treatment. Excessive amounts of polymer gels and other insoluble residues may significantly reduce fracture conductivity. Therefore, there is a need for a fracturing fluid and a method of treating a subterranean formation which would result in good fracture conductivity.